1. Field of the Invention
The present invention relates to a technique for demodulating a composite signal of a PAL (Phase Alternating Line) system and particularly to a technique for extracting a carrier chrominance signal from the composite signal of the PAL system.
2. Description of Related Art
The PAL system is known as one of video systems using a composite signal. In the PAL system, by quadrature two-phase modulating a color subcarrier with two chrominance signals (U and V signals), a carrier color signal (C signal) is produced and then transmitted. In order to correct for phase distortion inflicted during transmission due to transmission distortion or the like, the phase of the color subcarrier for the V signal is 180° inverted on a per line basis to produce the color signal C. Further, in order to prevent interference between lines, the frequency of the color subcarrier is selected such that the phase of each line is shifted by 90° relative to the previous line, that is, the phase is shifted by 180° every two lines.
Hence, in the composite signal of the PAL system (hereinafter called a PAL signal), as shown in FIG. 10, the phase of the C signal differs by 180° between line (n) and line (n−2) preceding by two lines in the same frame, and the phase of the color subcarrier for the V signal differs by 180° between line (n) and the preceding line (n−1). That is, in the same frame, the phase of the color subcarrier for the V signal is shifted by 180° every line, and the phase of the C signal is shifted by 180° every two lines.
Further, comparing the same lines (n) of different frames in phase, as shown in FIG. 11, the phase of the C signal for line (n) differs by 180° between frame (m) and frame (m−2) preceding by two frames, and the phase of the color subcarrier for the V signal for line (n) differs between frame (m) and the preceding frame (m−1). That is, for the same numbered line, the phase of the color subcarrier for the V signal is shifted by 180° every frame, and the phase of the C signal is shifted by 180° every two frames.
In video apparatuses such as receivers for the PAL signal and video recorders for recording the PAL signal on a record medium, when demodulating the PAL signal, Y/C separation is performed to separate the Y signal and the C signal with correcting the PAL signal for phase distortion. Techniques of Y/C separation include two-dimensional Y/C separation using the characteristic of the PAL signal shown in FIG. 10 and three-dimensional Y/C separation using the characteristic of the PAL signal shown in FIG. 11.
As to the three-dimensional Y/C separation, a method is known which takes the addition/subtraction of the composite signal for a frame of interest and the composite signal for the frame preceding by two frames the frame of interest. For example, for frame (m), Y/C separation is performed by taking the addition/subtraction of the composite signals for frame (m) and frame (m−2), and for frame (m+2), Y/C separation is performed by taking the addition/subtraction of the composite signals for frame (m+2) and frame (m).
With this method, the problem of cross color interference or cross luminance interference (hereinafter simply called cross interference) may occur. Description will be made of an example case where for only frame (m) of five frames (m−2) to (m+2) the C signal differs, that is, only the C signal of frame (m) is irrelevant to the C signals of the other frames. Because Y/C separation for frame (m) is performed by taking the addition/subtraction of the composite signals for frame (m) and frame (m−2), the C signal for frame (m) obtained by Y/C separation has the C signal of irrelevant frame (m−2) mixed therein, which means the occurrence of cross interference. Likewise, because Y/C separation for frame (m+2) is performed by taking the addition/subtraction of the composite signals for frame (m+2) and frame (m), the C signal for frame (m+2) obtained by Y/C separation has the C signal of irrelevant frame (m) mixed therein, which means the occurrence of cross interference.
In the Japanese Unexamined Patent Application Publication No. H08-18997 (hereinafter called reference 1), there is disclosed a technique to solve this problem. In the technique of reference 1, for frame (m) of interest, the preceding frame (m−1), and frame (m−2) preceding by two frames, BPF that passes the band of the C signal is performed to obtain three BPF output signals, and only when there is a correlation between these three BPF output signals, the BPF output signal for frame (m−1) or frame (m−2) selected by a signal selector is subtracted from the BPF output signal for frame (m) of interest, thereby extracting the C signal for frame (m) of interest. In contrast, when there is no correlation between the three BPF output signals, the BPF output signal for frame (m) of interest itself is adopted as the C signal for frame (m) of interest. In this way, cross interference in Y/C separation is avoided.
As to the two-dimensional Y/C separation, a method is known which takes the addition/subtraction of the composite signal for a line of interest and the composite signal for the line preceding by two lines the line of interest. For example, for line (n), Y/C separation is performed by taking the addition/subtraction of the composite signals for line (n) and line (n−2), and for line (n+2), Y/C separation is performed by taking the addition/subtraction of the composite signals for line (n+2) and line (n).
It is thought that the three-dimensional Y/C separation and the two-dimensional Y/C separation are suitable for still images and moving images respectively. In Japanese Unexamined Patent Application Publication No. 2005-277562 (hereinafter called reference 2), there is disclosed a technique in which combined Y/C separation of the two-dimensional Y/C separation and the three-dimensional Y/C separation is performed to suppress degradation in image quality when switching between still images and moving images.
In this technique, the three-dimensional Y/C separation and the two-dimensional Y/C separation are performed on the PAL signal while detecting motion represented in the video signal. The Y signal and C signal separated by the three-dimensional Y/C separation and the Y signal and C signal separated by the two-dimensional Y/C separation are mixed respectively such that as the motion becomes larger, the proportion of the signals obtained by the two-dimensional Y/C separation becomes larger, thereby realizing motion-adaptive Y/C separation.
The U and V signal components are extracted from the C signal obtained by Y/C separation (U/V separation), and color demodulation is performed on the extracted U and V signal components. Hereinafter, in order to distinguish signals before the color demodulation and signals after the color demodulation, the U and V signal components before the color demodulation are called carrier chrominance signals (a carrier U signal or u signal and a carrier V signal or v signal), and the U and V signal components after the color demodulation are simply called chrominance signals (U and V signals).
A method of the U/V separation is known which, utilizing the characteristic of the PAL signal shown in FIG. 10, takes the addition/subtraction of the C signal for a line of interest and the C signal for the line preceding the line of interest in the same frame, thereby separating two carrier chrominance signals for the line of interest.
With this method, there is the problem that, if a color boundary exists at a position in a vertical direction, the color of a line on the color boundary may not be its original color. For example, as shown in FIG. 12, where the color of line (n−2) and line (n−1) is magenta and the color of lines (n) to (n+2) is green, the addition/subtraction of the magenta of line (n−1) and the green of line (n) is taken in extracting carrier chrominance signals for line (n), and hence the carrier chrominance signals extracted for line (n) represent white.
In Japanese Unexamined Patent Application Publication No. 2002-58043 (hereinafter called reference 3), there is disclosed a technique to solve this problem. In this technique, for line (n) of interest and two lines each of before and after it, i.e. lines (n−2), (n−1), (n+1), (n+2), in the same frame, BPF that passes the band of the C signal is performed to obtain five BPF output signals C0, C1, C2, C3, C4, and a correlation between C0 and C2 and a correlation between C2 and C4 are obtained. A weighted average of the C1 and −C3 is taken in such a proportion that if the correlation between C0 and C2 of the two correlations is larger, the proportion of C1 is made larger and that if the correlation between C2 and C4 is larger, the proportion of −C3 is made larger, to obtain a color phase distortion correcting color signal N. Then by taking the addition/subtraction of the BPF output signal C2 for line (n) of interest and the color phase distortion correcting color signal N, carrier chrominance signals for line (n) of interest are separated. Applying this technique to an example frame shown in FIG. 12, since there is no correlation between the BPF output signals for lines (n−2), (n) and there is a correlation between those for lines (n), (n+2), the BPF output signal for lines (n+1) is used as the color phase distortion correcting color signal N. Therefore, the carrier chrominance signals for line (n) separated by taking the addition/subtraction of the BPF output signal for line (n) and the color phase distortion correcting color signal N, represent its original color.
Here the results of applying the U/V separation technique of reference 3 to the frame shown in FIG. 13 will be discussed. In the frame shown in FIG. 13, of line (n) of interest and two lines each of before and after it, i.e. lines (n−2), (n−1), (n+1), (n+2), only line (n) of interest is red in color and the other four lines are blue in color. When applying the U/V separation technique of reference 3 to this frame, the color phase distortion correcting color signal N for line (n) of interest is a weighted average of the BPF output signals for lines (n−1), (n+1) and hence is blue. Thus, the carrier chrominance signals for line (n) of interest U/V-separated by taking the addition/subtraction of the BPF output signal of line (n) of interest and the color phase distortion correcting color signal N, represent a color having blue mixed therein and not its original red. That is, with the U/V separation technique of reference 3, there is the problem that U/V separation cannot be correctly performed for a line of interest which differs in color from the lines before and after it.
The same problem occurs with a technique which performs U/V separation for a line of interest by taking the addition/subtraction of the C signal for the line of interest and the C signal for the preceding line.
Here the case of applying the technique of reference 1 to video shown in FIG. 14 will be discussed. In the example of FIG. 14, in each of frames 1 to 5, only line (n) of interest is red and the other lines are blue. Where Y/C separation is performed for frame 3 by the technique of reference 1, the BPF output signal of frame 2 or 3 is produced as the color phase distortion correcting color signal because there is a color correlation between frames 2, 3, 4, and the C signal for frame 3 is separated. However, in the U/V separation of the C signal obtained in this way, whichever of the above U/V separation techniques is used, the carrier chrominance signals for line (n) of interest in frame 3 represent a color having the color of line (n−1) or (n+1) mixed therein and not its original red. That is, also with the technique of reference 1, there is the problem that U/V separation cannot be correctly performed for a line of interest which differs in color from other lines in the same frame because a color correlation exists between frames on a corresponding line basis.